1. Field of the Invention
The invention relates to an electromagnetic relay, and more particularly to a structure of an electromagnetic relay having a resistivity to surge, that is useful for an apparatus receiving a surge such as lightning.
2. Description of the Related Art
A typical conventional relay has a basic structure that comprises an electromagnetic driving unit, a movable contact spring member and fixed contact terminals. A conventional polarized relay having dipoles of a magnetic will be described with reference to FIG. 1. The conventional polarized relay is accommodated in an enclosure member 8. The polarized relay has an electromagnetic driving unit that comprises an U-shaped core 1, a spool 2 surrounding the core 1, a coil 3 winding the spool 2, a permanent magnet 4 having magnetic dipoles N and S poles and coil terminals 9A and 9B projecting through the enclosure member 8. The polarized relay also has a movable contact unit that comprises an armature 5, an insulator 6 and a movable contact spring member 7. The armature 5 has a center convex portion that is kept in contacting with a polar face of the permanent magnet 4 and having opposite end portions that are alternatively contact with and apart from tops of the U-shaped core 1. The movable contact spring member 7 is mechanically connected to or electrically separated from the armature 5 through the insulator 6. The polarized relay also has fixed contact terminals 8B and 8M provided around the electromagnetic driving unit. The fixed contact terminals 8B and 8M have a bottom projected downwardly from a bottom face of the enclosure member 8 and a top portion located within the enclosure member 8. The movable contact spring member 7 shows a rotation about its center portion caused by or together with the rotation of the armature 5 so that its opposite ends are alternately contact with or apart from the tops of the fixed contact terminals 8B and 8M. The relay further has a movable contact terminal 10 projected downwardly from a bottom face of the enclosure member 8.
When the electromagnetic relay is applied to apparatus in the fields of communications and households, it is required to protect the apparatus from any surges due to interferences among a thunder surge, commercial power lines and communication lines. Achievement of such protection requires the coil terminals 9A and 9B, the fixed terminals 8B and 8M, the movable contact spring member 7 and a movable contact terminal 10 to have resistivities against a relatively large surge voltage. The conventional relays have been engaged with serious problems the resistivities of the large surge voltages. When the relay receives a surge whose voltage waveform is illustrated in FIG. 2A, the relay shows electrical responses illustrated in FIGS. 2C and 2D wherein the relay may be regarded as a structure comprising three terminals A, B and C connected in series through two floating static capacitances C1 and C2. The terminal A represents the coil terminals 9A an 9B. The terminal B represents electrically floating members such as the core 1 and the armature 5. The terminal C represents the fixed contact terminals 8B and 8M and the movable contact terminal 10. Then, a surge voltage E0 applied between the terminals A and C is divided by the floating static capacitors C1 and C2. A surge voltage E1 between the terminals B and C is given by the following equation (1), while a surge voltage E2 between the terminal A and B is given by the following equation (2). EQU E1=E0.times.C2/(C1+C2) (1) EQU E2=E0.times.C1/(C1+C2) (2)
From the above equations, it could readily be understood that if the values of the two floating static capacitances C1 and C2 are unbalanced or have a large difference from each other, then the surge voltages E1 and E2 are unbalanced or have a large difference. Normally, the floating static capacitance C2 tends to be larger than the floating static capacitance C1 as the coil 3 is so winded around the extremely thin spool 2 as to have a possible large winding density in order to improve an efficiency in electromagnetic conversion. The floating static capacitance C1 is mainly defined by the insulator 6 that mechanically connects the movable spring member 7 to the armature 5. In view of securing a necessary length of the movable contact string member or a voltage resistivity particularly in scaling down, it is difficult to enlarge the floating static capacitance C1. The most of the surge voltage E0 is applied between the terminals B and C. This may provide a breakdown voltage between the terminals B and C. The breakdown between the terminals B and C may further provide a breakdown voltage between the terminals A and B. Then this results in a further breakage between the terminals A and B. Consequently, this results in a breakage between the terminals A and C. To prevent this problem if the distance between the armature and the movable contact spring member is increased or the insulator is inserted between the armature and the movable contact spring member to improve the voltage resistivity between the terminals A and B, then this may act as a bar to scaling down of the electromagnetic relay or a bar to an achievement of high speed performance of the armature.
It has been required to develop a novel electromagnetic relay free from any problems.